It could be your over steeped, or squeezed your grains. There are a few threads on it, but it boils down to about the same as doing it to coffee/tea. By over steeping, or squeezing the grains, you release te)@_ (? I forget the word..) Get a good calibration on your thermometer to see if that's a possibility.
Second, although doesn't seem likely, is how long did you age it? This doesn't sound like a normal character of green beer, but a possibility. Sorry I can't shed more light on the issue. Hope you figure it out (and it's fixable)!